To improve the printability of paper and cardboard, one or more coat layers can be applied on these webs. The greater the number of coat layers, the better the quality of the coated sheet with the additional possibility of using coating furnishes of different compositions. However, such use of two different coat layers causes a significant increase in production costs, because the coat application is conventionally made in several steps and the applied coat is dried prior to the application of the next layer.
The coat can be applied either directly to the web surface using, e.g., a blade or rod coater, or alternatively, a transfer roll coater in which a coat film premetered on a transfer roll is applied in a roll nip to the web surface. Blade and rod coaters are characterized in that the coater seals the surface texture voids of the base web and smooths the surface, whereby the thickness of the coat layer varies according to the volume of the surface texture voids in the base web. The final result is a smooth coat having an uneven brightness distribution combined with difficult control of smooth absorption of the coat.
Transfer roll coating provides a coat with an essentially improved constant thickness, whereby easy control of coat absorption is attained, though sufficient smoothness properties are a problem, particularly with thicker grades of paper and paperboard. The transfer coat also achieves a relatively even coverage whereby the coated web brightness of a base web with, e.g., a low initial brightness, can be improved essentially using this coating method. Furthermore, since the transfer roll coating method dispenses with a doctor blade running in contact with the base web to be coated, the method offers excellent runnability with respect to interruptions in the coating process.
Two different approaches have been applied to utilize the advantageous properties of both coating methods.
Transfer roll coating is used widely in precoat application. This method can improve the coating result through the increased total coat weight in which a single doctored coat layer can assume sufficient coat smoothness. Conventionally, the precoat is dried prior to the subsequent final coat application, which typically is applied using a doctor blade coater. Also known in the art from U.S. Pat. No. 2,937,955 is doctor blade application directly onto a semidry transfer precoat. A precondition to the use of this method is that the precoat has already settled or set so much that it can take the final coat application by means of the doctor blade coater. Setting of the coat can be speeded by, e.g., partial drying of the precoat in situations where the coat has not become sufficiently dewatered through absorption under pressure in the coating nip and due to moisture migration from the coat during the web travel over the free distance between the precoater and the final coater units.
The web travel delay and coat setting time occurring between the transfer precoater and the doctor blade final coater can be extended by, e.g., increasing the web travel distance between the coater units, thus achieving sufficient setting and dryness of the precoat through the extended moisture absorption time for the subsequent doctor blade coat application step. U.S. Pat. No. 5,340,611 discloses a method in which the amount of coat applied in the transfer roll coating step is kept so small that the doctor blade of the knife applicator device cannot scrape the precoat away. However, such a method fails to achieve the maximum precoat weight possible by means of a transfer roll coater, and the first precoat layer remains very thin. According to the method, the smoothing element of second applicator device can be a levelling rod instead of a doctor blade.
In pilot-scale test runs of this paperboard coating method, not even increasing the delay between the coating steps could make the precoat set at 10 g/m.sup.2 coat weight sufficiently to avoid partial scraping-away of the already applied precoat in the subsequent doctor blade applicator. As the web speed is fixed by the speed at the coat transfer nip while the subsequent blade applicator causes a braking action on the web, a problem was also caused by the tendency of the web to produce a bag or slack in front of the smoothing blade of the applicator device resulting in web breaks. Hence, the method was not found practicable. The frequency of web breaks is further increased by the fact that the blade application step, which causes appreciably higher stress on the web than the preceding transfer roll coating step, is performed on a web of high moisture, whereby the web strength is obviously much lower than that of a dry web.
Another approach to the utilization of the special characteristics of transfer and blade application steps is disclosed in finnish patent application Ser. No. 941,803 of Valmet Corporation wherein the coater unit has a design permitting the use, according to the specifications of the final product and/or runnability criteria, of either a transfer or blade applicator as alternative methods yet employing the same coat dryer for both coating methods. Because this apparatus fails to utilize the benefits of two-layer coating, its principal benefit is to increase the flexibility of production, but not to improve the quality of the end product over comparable one-layer coating methods.
However, particularly thicker grades of paper and paper-board present a need for combining the benefits of both the transfer and the blade coating methods. Yet, due to lack of sufficient footprint and investment capital, intermediate drying of the precoat is not always possible. In such a case the only remaining alternative is the above-described type of wet-on-wet coating, in which the present methods fail to provide sufficiently heavy total coat thicknesses in which the high opacifying power of the transfer roll coating method and the good smoothness given by blade application could be utilized in an optimal manner.